Higman Chris Gasification (Газификация угля)

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Gasification

20–70 bar and at high temperatures of at least 1400°C. Entrained-flow gasifiers

have become the preferred gasifier for hard coals and have been selected for the

majority of commercial-sized IGCC applications.

In entrained-flow gasifiers the fine coal particles react with the concurrently

flowing steam and oxygen. All entrained-flow gasifiers are of the slagging type,

which implies that the operating temperature is above the ash melting point. This

ensures the destruction of tars and oils and, if appropriately designed and operated,

a high carbon conversion of over 99% although some water-slurry feed plants do

not achieve this. Moreover, entrained-flow gasifiers produce the highest quality

synthesis gas because of the low methane content. Entrained-flow gasifiers have rel-

atively high oxygen requirements, and the raw gas has a high sensible heat content.

The various designs of entrained-flow gasifiers differ in their feed systems (dry-coal

feed in a high-density fluidized state or coal-water slurries), vessel containment for

the hot conditions (refractory or membrane wall), configurations for introducing the

reactants, and the ways in which sensible heat is recovered from the raw gas. The

two best-known types of entrained-flow gasifiers are the top-fired coal-water-slurry

feed gasifier, as used in the Texaco process and the dry coal feed side-fired gasifier

as developed by Shell and Krupp-Koppers (Prenflo). Furthermore, there is the dry

coal feed top-fired Noell gasifier. Simple sketches of these three reactor types are

Table 5-7

Characteristics of Important Entrained-Flow Processes

Process Stages Feed Flow Reactor Wall Syngas Cooling Oxidant

Koppers-

Totzek 1 dry up jacket syngas cooler oxygen

Shell SCGP 1 dry up membrane gas quench and

syngas cooler

oxygen

Prenflo 1 dry up membrane gas quench and

syngas cooler

oxygen

Noell 1 dry down membrane water quench

and/or syngas

cooler

oxygen

Texaco 1 slurry down refractory water quench or

syngas cooler

oxygen

E-Gas 2 slurry up refractory two-stage

gasification

oxygen

CCP

(Japan)

2 dry up two-stage

gasification

air

Eagle 2 dry up membrane two-stage

gasification

oxygen

Gasification Processes

111

given together with temperature profiles for both the coal/char and the gas in

Figures 5-13, 5-14, and 5-15. Some gasifiers (e.g., E-gas) use two stages to improve

thermal efficiency and to reduce the sensible heat in the raw gas and to lower the

oxygen requirements. In the present coal-water slurry-feed gasifiers, a substantial

part of the reactor space is used to evaporate the water of the slurry. This is reflected

in the temperature profile of this gasifier (see Figure 5-13).

Contrary to the moving-bed and fluid-bed processes, virtually all types of coals

can be used in these processes, provided they are ground to the correct small size.

The coals may be heavily caking and may range from sub-bituminous coals to

anthracite. Browncoal and lignite can in principle be gasified, but for economic

reasons this is not very attractive because of the ballast of inherent moisture that has

to be evaporated and heated to the high temperatures required. High-ash coals are

also not selected by preference, because all the ash has to be melted and that also

constitutes thermal ballast to the gasifier. Coals with very high ash melting points

are generally fluxed with limestone in order to lower the ash melting point and

hence the operating temperature. This improves the process efficiency, reduces the

oxygen consumption, and enables the use of a refractory-lined reactor.

Currently, most entrained-flow gasifiers are single-stage gasifiers. The fuel is

introduced together with the blast via one or more burners. The blast is always pure

oxygen or a mixture of oxygen and steam.

The ash produced in entrained-flow gasifiers is similar to that of the slagging

moving-bed gasifiers and consists of the same fine, black, inert, gritty material.

Reactor Modeling

The upflow reactors, as employed in the Shell Coal Gasification Process (SCGP)

and Prenflo reactors, can be essentially considered as continuously stirred tank reactors

2500 12501000750500 1500

COAL/WATER

SLURRY

OXYGEN

GAS & SLAG

TEMPERATURE [°C]

OXYGEN

COAL

SLAG

GAS

Figure 5-13.

Figure 5-13.Figure 5-13.

Figure 5-13. Top-Fired Coal-Water Slurry Feed Slagging Entrained-Flow Gasifier

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Gasification

(CSTRs). The reason is the very large recirculation inside the reactor caused by

small temperature differences and the way the reactants are introduced. Contrary to

the fluid-bed reactors, the carbon conversion is almost 100% because most of the

ash leaves the reactor as a slag that is very low in carbon. Moreover, measures can

be taken to ensure that large carbon particles tend to remain longer in the reactor.

This can be accomplished, as in the Noell gasifier, by introducing some swirl in the

top burner or by tangential firing as, for example, in the EAGLE gasifier. The

largest coal particles are thus preferentially deposited on the liquid slag flowing

2500 12501000750500 1500

TEMPERATURE [°C]

GAS & SLAG

STEAM,

OXYGEN OR

AIR

COAL

STEAM, OXYGEN

OR AIR

SLAG

GAS

Figure 5-14.

Figure 5-14.Figure 5-14.

Figure 5-14. Top-Fired Dry-Coal Feed Slagging Entrained-Flow Gasifier

2500 12501000750500 1500

TEMPERATURE [°C]

COAL

OXYGEN/STEAM

DRY COAL +

OXYGEN/STEAM

SLAG

GAS

Figure 5-15.

Figure 5-15.Figure 5-15.

Figure 5-15. Side-Fired Dry-Coal Feed Slagging Entrained-Flow Gasifier

Gasification Processes

113

vertically downwards along the reactor wall. The coal particles having a lower

density than the slag will float like “icebergs” on the slag. The velocity of this slag

layer is much lower than of the gas in the reactor, and thus the ideal situation is

obtained where the larger coal particles get the longest residence time. Careful

design is important, as too much swirl causes reverse flow in the center of the

reactor and can lead to unwanted situations.

Modeling of the second nonslagging stage of the E-Gas reactor is also not simple

because of the evaporation of water and the pyrolysis reactions. The first slagging

stage with the side introduction of coal-water slurry and recycled char in the E-Gas

process can again be described as a CSTR.

CFD modeling of entrained-flow reactors has been initiated, and the first published

results are encouraging (Bockelie et al. 2002).

5.3.1 General Considerations

Dry-Coal Feed Gasifiers

As discussed in Chapter 2, dry-coal feed gasifiers have the advantage over coal-

water slurry feed gasifiers in that they can operate with almost the minimum amount

of blast. This implies in practice that they have a 20–25% lower oxygen consump-

tion than coal-water feed gasifiers. Also, as shown in Section 2.4, dry-coal feed

entrained-flow gasifiers have in principal an additional degree of freedom that makes

it possible to better optimize the synthesis gas production. Moreover, it is possible to

adjust the H

/CO ratio slightly. In practice, operation at a CO

content of the gas of

0.5–4 mol% and a temperature of 1500°C is generally adhered to.

Single-Stage Gasifiers. In particular, the single-stage entrained-flow gasifiers yield

a high gas purity with only traces of hydrocarbons and with a CH

content of well

below 0.1 mol%. Together with the low CO

and high carbon conversion, this

ensures that almost all carbon in the feed is converted into CO, and hence a non-

selective acid gas removal can be employed, as the H

S/CO

is such that the combined

acid gas may be routed directly to the Claus plant sulfur recovery. Details about the

gas treating will be discussed in Chapter 8.

Examples of single-stage dry-coal feed gasifiers are the SCGP process, the Prenflo

process, and the Noell process. The dry-coal-feed process SCGP is used in a

250 MW IGCC plant in Buggenum, The Netherlands, and the Prenflo process in

a 300 MW IGCC plant in Puertollano, Spain. Noell has a 600 t/d plant operating on

a variety of solid and liquid feedstocks in Schwarze Pumpe, Germany.

Process Performance. One of the most striking features of single stage dry-coal

entrained-flow slagging gasifiers is that the gas composition is very insensitive

to the coal quality. In the case of low-rank coals and high-ash coals, however,

the gas yields suffer because of the ballast of water and ash, respectively. The

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Gasification

performance of a variety of coals is given in Table 5-8. The coal analyses are

those of Table 4-4.

Two-Stage Gasifiers. An improvement in the process efficiency can be obtained by

adding a second nonslagging stage to the first slagging stage of an entrained-flow

gasifier.

Based on the standardized, idealized conditions of Appendix E, present state of the

art single-stage pressurized entrained-flow gasifiers have an efficiency of 50% LHV

for IGCC applications (see data in Table 5-9). The process efficiency of these gasifiers

can be increased by the introduction of a second nonslagging stage as applied in the

CCP or EAGLE processes. The second stage results in a higher cold gas efficiency

and lower oxygen consumption. Efficiencies are 50 and 50.9% for the one- and two-

stage process-based IGCC, respectively, when operating with a dry-coal feed system

(see Table 5-9). (The calculation basis for this data is given in Appendix E.)

This is achieved by operating the first stage under high temperature slagging

conditions with only part of the reactants, and then adding the remainder in a second

stage where the hot gas drives the endothermic reactions in the second, nonslagging

stage. In practice, current processes all operate the first stage with a deficit of coal

with a second stage coal feed. In principle, however, alternative staging concepts

could be used, e.g. with steam addition as the second stage. Each staging concept

has its advantages and disadvantages. With a coal feed second stage there is the fact

that a certain amount of tars will inevitably leave the reactor with the syngas. The

extent to which this constitutes a real problem depends on the feedstock quality and

the actual outlet temperature of the reactor. With steam addition in the second stage,

the fuel is present as unconverted char from the first stage so that the syngas would

be tar free. On the other hand, in the context of a dry-feed system, this requires

considerably more steam than for a single stage reactor. This increases the amount

of process condensate to be handled and degrades some of the sensible heat in the

gasifier exit gas to condensing temperature levels.

Irrespective of the choice staging arrangement, there are a number of interesting

aspects to be considered. The lower temperature of the second stage will require

more residence time than for a single stage gasifier and in practice one will have to

reckon with some carbon carry over in the syngas. On the other hand precisely these

lower temperatures would allow the use of a refractory wall in the second stage,

which represents a significant cost saving compared with the membrane wall used in

some single stage designs without the exposure to very high temperatures in other

singlestage designs.

The second stage is nonslagging. The particulate matter in the syngas contains

unreacted carbon and dry ash. This is removed from the gas downstream of the syngas

cooler and recycled to the first stage. In this way almost all the ash is removed from

the system as slag.

The overall effect is that a slagging gasifier is obtained in which the oxygen consump-

tion is almost as low as for a gasifier operating at the temperature at which the gases

leave the second nonslagging stage. This has the following process advantages:

Gasification Processes

115

Table 5-8

Performance of Various Types of Coals in Dry-Coal Entrained-Flow Gasifiers

Coal

Gas Analysis of Dry

Gas, Mol% Miscellaneous

Country Region Classification CO H

CO + H

ton maf coal

/Nm

CO + H

kg steam/Nm

CO + H

Germany Rhein Browncoal 61 29 8 1 1 0.2 965 0.33 0

USA North Dakota Lignite 62 26 10 1 1 0.1 935 0.36 0

USA Montana Sub-bituminous 63 34 1 1 1 0.4 1950 0.26 0.06

USA Illinois Bituminous 61 35 1 1 1 1.5 2030 0.25 0.09

Poland typical Bituminous 58 39 1 1 1 0.2 2290 0.20 0.15

S. Africa typical Bituminous 64 33 1 1 1 0.3 2070 0.26 0.09

China Datung Bituminous 66 31 1 1 1 0.2 2060 0.27 0.09

India typical Bituminous 62 33 2 1 1 0.5 1730 0.31 0

Australia typical Bituminous 62 34 1 1 1 0.3 2100 0.26 0.07

Germany Ruhr Anthracite 65 31 1 1 1 0.2 2270 0.26 0.13

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Gasification

1. No gas quench and a lower syngas cooler duty.

2. A higher cold gas efficiency and a 20% lower oxygen consumption.

The methane content of the gas will slightly increase compared with the single stage

gasifier and so will the CO

content.

A drawback of incorporating a second-stage to a dry-feed gasifier is the added

complexity and the higher steam consumption. The extra efficiency is about one

percentage point (see Table 5-8). But there are also other advantages, such as much

less quenching and a lower cost syngas cooler.

Another process advantage that can be incorporated in all slagging processes is a

boiling water slag bath where the steam generated can be used for example as pro-

cess steam or as a quench medium. Further, the slag bath could be used as a sour

water stripper.

Coal-Water Slurry-Fed Gasifiers

The big advantage of coal-water slurry feed gasifiers is the more elegant method of

pressurizing the coal. Lock hoppers as used in dry-coal feed gasifiers are costly and

bulky equipment with complex valve systems that have to provide a gas-tight block

in a dusty atmosphere. Pumping a coal-water slurry is not a simple operation either,

but it is definitely less complex than lock-hoppering. In addition, the practical

ultimate pressure for dry-pulverized-coal lock hoppers is about 50 bar, whereas for

coal-water slurry pumps the pressure could, in principle, be as high as 200 bar.

As there is always a surplus of gasifying agent to be added to the gasifier, there is

less benefit in adding a nonslagging second stage. Only when coal is added to the

second stage, as is done in the E-Gas process, are part of the disadvantages of

having so much water in the feed eliminated.

Table 5-9

IGCC Efficiencies for Various Entrained-Flow Gasifiers

Process Feed

Syngas

Cooling

Gasifier

Conditions

IGCC Efficiency,

%LHV

Slurry feed water quench 64 bar 1500°C 37.8

Slurry feed gas quench 64 bar 1500°C 43.6

Slurry feed 320°C gas quench 64 bar 1500°C 48.8

Dry feed gas quench 32 bar 1500°C 50.0

Dry feed two-stage

gas quench

32 bar 1500/1100°C 50.9

Note: Efficiencies are based on the standardized, idealized conditions of Appendix E.

Gasification Processes

117

Single-Stage Gasifiers. In a single-stage coal-water slurry-feed gasifier all the water

in the slurry must be evaporated and raised to the full outlet temperature of the slag-

ging operation. This imposes a considerable penalty on the cold gas efficiency of the

process to offset against the simplified feed system. This is also reflected in a higher

oxygen consumption compared with a dry-feed system. Furthermore space in the

reactor is required for the evaporation process.

The high water vapour content of the hot, raw syngas also influences its composi-

tion. The CO shift reaction is driven further to the right resulting in a higher H

/CO

ratio and higher CO

content than the equivalent dry-feed gasifier. Whether this is

important or not will depend on the application under consideration. Additionally

the methane content will be even lower although if the potential for higher operating

pressures is exploited, this would tend to increase the methane content. However the

methane content of all single-stage entrained-flow gasifiers is so small that this is

unlikely to be of consequence.

Two-stage gasifiers. The main issues surrounding the staging of coal-water slurry-feed

gasifiers are similar to those for dry feed gasifiers. The efficiency of a two-stage

gasifier is higher than that of a single-stage process for the same reasons as for the

dry feed case. The disadvantages of a higher steam consumption described for dry

feed gasifiers are however not applicable, since the amount of steam is dictated by

the coal/water ratio in the slurry, which is not affected by the staging arrangement.

5.3.2 The Koppers-Totzek Atmospheric Process

Just as with moving-bed and fluid-bed processes, the first entrained-flow slagging

gasification process operated at atmospheric pressure. The atmospheric pressure

Koppers-Totzek (KT) process was developed in the 1950s, and commercial units

were built in Finland, Greece, Turkey, India, South Africa, Zambia, and elsewhere,

mostly for ammonia manufacture. The South African unit has been reported as

achieving a 95% availability (Krupp-Koppers 1996). In recent years no new units of

this type have been built.

Process Description

The KT reactor features side-fired burners for the introduction of coal and oxygen,

a top gas outlet, and a bottom outlet for the slag. The early units had a capacity of

5000 Nm

/h and featured two diametrically opposed burners that were situated in

horizontal truncated cones (see Figure 5-16). Later units featured four burners that

increased the maximum capacity to 32,000 Nm

/h. The gas leaving the top of the

gasifier at about 1500°C is quenched with water near the top of the reactor to a

temperature of about 900°C so as to render the slag nonsticky before it enters a water

tube syngas cooler for the production of steam. The reactor has a steam jacket to

protect the pressure shell from high temperatures. A significant portion of the sensi-

ble heat is transformed into low-pressure steam in the jacket, which represents a

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Gasification

considerable energy penalty for the process. The burners are of the premix type

which means that the velocity in the burners must be quite high in order to avoid

flash backs. For pressurized burners, premixing is considered too dangerous and has

therefore never been applied. The slag is quenched and granulated in a water bath

underneath the reactor. The water in the slag bath also provides a water seal to avoid

gas escaping via the bottom of the reactor.

5.3.3 Shell Coal Gasification Process (SCGP) and Prenflo Process

The origin of both these processes goes back to the Koppers-Totzek process. Shell

and Koppers jointly developed a pressurized version of the process, and in 1978

they started to operate a 150 t/d gasifier in Harburg, Germany. The main interest for

Koppers was to have a better process available for the production of syngas. For

Shell the main interest was at the time the production of synthetic fuels from coal by

HP-

STEAM

BFW

RAW SYNGAS

RADIANT

COOLER

COAL

LP STEAM

OXYGEN

SLAG BATH

INJECTION

WATER

COAL

TUBULAR

COOLER

SCRUBBING

TOWER

CLEAN GAS

GASIFIER

REFRACTORY

Figure 5-16.

Figure 5-16.Figure 5-16.

Figure 5-16. Koppers-Totzek Gasifier

Gasification Processes

119

the route gasification and Fischer-Tropsch synthesis. After the joint Harburg unit,

Koppers and Shell decided to develop the process further along separate routes.

Subsequently, Shell built a 250 t/d demonstration unit in Houston, and Krupp-

Koppers a 48 t/d unit in Fürstenhausen, Germany. Based on the work in these units,

two commercial plants were built as part of IGCC power station. In 1994 a 2000 t/d

Shell gasification unit was built for Demkolec in Buggenum in The Netherlands

using internationally traded coal as original feedstock, and in 1997 Krupp-Koppers

built a 3000 t/d Prenflo unit for Elcogas in Puertollano in Spain using a blend of

high-ash coal and petcoke as feedstock. Currently, Shell has various projects under-

way in China.

Process Description

The SCGP and Prenflo processes, which are very similar (Anon 1990), feature an even

number (typically four, see Figure 5-17) of diametrically opposed burners in the

side-wall at the bottom of the reactor through which the pulverized coal is introduced

in a dense phase using an inert gas as carrier gas (van der Burgt and Naber 1983).

The small niches in which the burners are placed are a remnant of the truncated

MEMBRANE

WALL

OXYGEN

MP STEAM

HP STEAM

QUENCH GAS

BLOWER

TO GAS

TREATMENT

FLY SLAG

BFW

SLAG

PULVERIZED

COAL

REFRACTORY

Figure 5-17.

Figure 5-17.Figure 5-17.

Figure 5-17. Shell Coal Gasification Process (Source: Adapted from Koopman,

Regenbogen, and Zuideveld. Used with permission from Shell. 1993)